The present invention relates to a Coriolis-type mass flowmeter. More particularly, the invention relates to such a flowmeter which comprises dual parallel flowtubes and at least one brace bar connected between the flowtubes for decoupling extraneous modes of vibration from the desired the driven and Coriolis modes of vibration.
Coriolis mass flowmeters typically comprise at least one flowtube which is connected to a section of process piping that contains a fluid to be measured, one or more force drivers for vibrating the flowtube in one of its natural modes of vibration, such as its first bending mode of vibration, and a number of motion sensors for detecting the vibrating motion of the flowtube. As the fluid flows through the vibrating flowtube it generates Coriolis forces that cause the flowtube to deform into a unique and characteristic shape, which is commonly referred to as the xe2x80x9cCoriolis deflectionxe2x80x9d. Thus, in operation the flowtube is subjected to a xe2x80x9cdrivenxe2x80x9d mode of vibration, which is generated by the force drivers, and a xe2x80x9cCoriolisxe2x80x9d mode of vibration, which is generated by the Coriolis forces. As is well understood by those of ordinary skill in the art, the mass flow rate and certain other properties of the fluid can be determined from the Coriolis deflection of the flowtube, which is measured by the motion sensors.
In the field of mass flow measurement, dual tube Coriolis mass flowmeters are often preferred for their accuracy and dependability. These flowmeters typically comprise two identical parallel flowtubes, an inlet manifold which is connected between the process piping and a first end of each flowtube, and an outlet manifold which is connected between the process piping and a second end of each flowtube. The flowtubes can have various configurations, including S-shaped and U-shaped, and each flowtube lies in a plane that is parallel to the plane of the other flowtube when the flowmeter is assembled. Furthermore, the force drivers are mounted so as to vibrate the flowtubes toward and away from each other in the Z direction, which is defined herein as being perpendicular to the planes of both flowtubes. Consequently, the flowtubes vibrate 180 degrees out of phase with each other in both the driven and Coriolis modes of vibration.
In addition to the desired driven and Coriolis modes of vibration, the flowtubes can be subjected to extraneous modes of vibration which are caused by, for example, pumps or other auxiliary equipment that are connected to the process piping. These extraneous modes of vibration, which can cause the flowtubes to vibrate either in-phase or out-of-phase with each other, can couple to the driven and Coriolis modes of vibration and thereby cause flow measurement errors. As used herein, the term xe2x80x9ccouplexe2x80x9d as between two modes of vibration means that the energy from one mode of vibration is linked to the second mode of vibration in such a way as to cause the excitation and response of that second mode of vibration. Therefore, it is desirable to prevent any energy exchange between the extraneous modes of vibration and the driven and Coriolis modes of vibration in order to minimize the potential flow measurement errors.
Brace bars have been employed in prior art dual tube Coriolis mass flowmeters to decouple certain extraneous modes of vibration from the desired driven and Coriolis modes of vibration. As described for example in U.S. Pat. No. 5,370,002, a brace bar is typically a flat plate having two holes through which the flowtubes are passed and secured. Two separate brace bars are usually connected to the flowtubes at spaced-apart locations, and the force drivers and motion sensors are positioned between the brace bars. Consequently, the pivot points for the driven and Coriolis vibrations are shifted to the brace bars, while the pivot points for certain of the extraneous vibrations remain at the inlet and outlet manifolds. Thus, the brace bars separate the frequency of these extraneous modes of vibration from the frequency of the driven and Coriolis modes of vibration and thereby decouple the extraneous modes from the driven and Coriolis modes.
However, certain lateral modes of vibration, that is, extraneous modes of vibration which are parallel to the planes of the flowtubes, can cause the normally round cross section of each flowtube to deform elliptically where the flowtube passes through the brace bar. Moreover, this elliptical deformation may cause traditional brace bars to stretch and compress during the vibration cycle of the lateral mode, which in turn can push and pull the centerlines of the flowtubes toward and away from each other. As a result, the flowtubes are caused to vibrate out-of-phase with each other in the Z-direction. Furthermore, since these extraneous vibrations emanate from brace bars, they can directly couple to the driven and Coriolis modes of vibration and result in flow measurement errors.
In accordance with the present invention, these and other limitations in the prior art are overcome by providing a brace bar for a Coriolis mass flowmeter having first and second generally parallel flowtubes which each extend between an inlet manifold and an outlet manifold and comprise respective first and second centerlines that lie in corresponding parallel first and second planes, at least one force driver for vibrating the first and second flowtubes in a natural mode of vibration and at least one motion sensor for detecting the vibrating motion of the flowtubes, the brace bar comprising a first stiffening member having a first opening through which the first flowtube is received and secured, a second stiffening member having a second opening though which the second flowtube is received and secured, and at least one bridge component which is connected to the first stiffening member proximate the first plane and the second stiffening member proximate the second plane.
In this manner, the first and second stiffening members will restrict the degree to which the first and second flowtubes can deform elliptically when subjected to lateral modes of vibration. In addition, the stiffening members are preferably connected only through the bridge component so that any elliptical deformation of the stiffening members, which may be caused by the elliptical deformation of the flowtubes, will not result in movement of the flowtubes in the Z-direction. Furthermore, since the bridge component is connected to the stiffening members only through the first and second planes of the flowtubes, the elliptical deformation of the flowtubes will not cause the bridge component to deform and displace the flowtubes in the Z-direction. Therefore, the brace bar of the present invention effectively decouples the lateral modes of vibration from the driven and Coriolis modes of vibration.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments.